The aug is a lot more comfortable to shoot. The FS2000 leaves me with a feeling of being punched in the cheek when I fire it. Mounting an optic that sat higher helped with that somewhat. The a3 is a little front heavy. I think I prefer the a1 style as it is lighter. If you like having rails the a3 has plenty of them. Both disassemble easily. The fs2000 uses standard ar15 mags, as in GI style. Pmags will not fit unless you modify the rifle by removing the mag well "wipes". The aug mags are pricey. They do have a nato version that uses ar15 mags, but its harder to get a hold of and costs a little more. The aug has 9mm conversion kits out there, but they are pricey. I like both but I like the aug more.
The Tavor is better than both. The controls are laid out better, it takes any AR mag, points great, offers conversion kits (soon). Light recoil, and many more things.
Id keep the Tavor over an a3 aug, but between the a1 and tavor it wouldbe a tougher choice.